Gas Piping, Pipe Sizing, and Connections
What the exam tests
Pipe sizing, materials, pressure regulation, sediment traps, leak testing, and code-compliant connections. Questions often require choosing a pipe size for a given load and length.
Core rules
- Low-pressure natural gas typically enters at 7 in. w.c. and is regulated to 3.5 in. w.c. manifold; propane enters at 11 in. w.c. with a 10–11 in. w.c. manifold.
- Size by the longest-length method: total Btu load, longest run, and allowable drop (often 0.5 in. w.c.). Fittings and valves add equivalent length.
- Black steel is standard; CSST must be bonded and installed per manufacturer; copper is common for propane but can suffer sulfide corrosion with natural gas.
- Install a sediment trap/drip leg at the appliance, a manual shutoff valve, a union, and a regulator where required. Apply pipe dope rated for gas to male threads only.
Worked example
A 100,000 Btu/hr furnace is 50 ft from the meter. At 0.5 in. w.c. drop, 1/2 in. steel is limited to roughly 40,000–50,000 Btu/hr at that length, so it is undersized. 3/4 in. steel carries about 100,000–120,000 Btu/hr at 50 ft, so 3/4 in. is the minimum.
Traps
Do not size by branch alone; include all appliances. Do not use a flame for leak testing. Do not omit the drip leg or install a shutoff downstream of the union. Remember that propane has more Btu per cubic foot, so it needs a smaller orifice and a different regulator spring.